In recent years, the demand for electrophotographic devices, such as copying machines and laser printers, has dramatically increased with development of the electrophotographic system and the requirements for their performance have become sophisticated. Generally, in an electrophotographic process, an electrostatic image (a latent image) is formed on a photosensitive material and then the latent image is developed using a toner, and thus a toner image is formed. The toner image is transferred to a recording medium such as paper and then is fixed by such means as heating.
In order to pass through such a process without any problems, the toner is first required to hold a stable amount of charge, and is secondly required to have good fixing property to paper. Moreover, since a device has a heater in its fixing part, the temperature rises in the device, and therefore, the toner is required not to suffer from blocking within a device.
Moreover, in addition to acceleration of size reduction, increase in speed, and improvement in image quality of the electrophotographic devices, improvement of a toner in low-temperature fixing property has strongly been demanded from the viewpoint of energy saving, namely, reducing energy consumption in the fixing step. When a toner image is fixed by a hot roll fixation system, a hot roll and a toner in a molten state come into contact with each other directly at the time of fixation. At this time, since there occurs a so-called “offset phenomenon” in which the toner transferred onto the hot roll stains the paper that is fed next, offset resistance is necessarily required. Therefore, it is necessary to develop low-temperature fixing property while maintaining offset resistance, and a toner having a wider working range, e.g., a fixing temperature range of 50° C. or more, has increasingly been required.
Toner binders have a great influence on toner properties like those mentioned above. Although there are known polystyrene resin, styrene-acrylic resin, polyester resin, epoxy resin, polyurethane resin, polyamide resin, etc., the polyester resin has recently attracted particular attention because it is easy to balance storage property and fixing property.
Conventionally, as a method for expanding the fixing temperature range of polyester resin, there has been studied a method using a non-linear polyester resin having a three-dimensional crosslinked structure prepared using tri- or more functional monomers (see, for example, Patent Document 1). The non-linear polyester resin disclosed in Patent Document 1 is superior in hot offset resistance and can develop a high maximum fixing temperature; however, it is not sufficient with respect to the level of low-temperature fixing property.
Then, as means for improving the low-temperature fixing property, there has been studied using a linear polyester resin composed of a divalent carboxylic compound and a dihydric alcohol compound (see, for example, examples in Patent Document 2). Although a linear polyester resin having no three-dimensional structure is superior in low-temperature fixing property, it is problematic in that a wide fixing temperature range cannot be secured because of its poor hot offset resistance.
Patent Document 3 proposes a toner using a product produced by making a polyester resin having unsaturated double bonds undergo a crosslinking reaction with a radical reaction initiator. However, even if this method can similarly prevent the offset phenomenon at high temperatures to some extent, it becomes difficult to attain low-temperature fixation because the fixation lower limit temperature is also raised simultaneously, and therefore, that method has not sufficiently met the demand for increasing the speed or saving energy, yet.
Moreover, toners using a reaction product of an isocyanate with a mixture of a resin for producing a macromolecules and a polyester resin have been proposed (Patent Documents 4 to 11). However, even if this method can similarly prevent the offset phenomenon at high temperatures to some extent, it becomes difficult to attain low-temperature fixation because the fixation lower limit temperature is also raised simultaneously and moreover, the uniformity of resin is impaired and the heat resistant storage property is also deteriorated, and therefore, that method has not sufficiently met the demand for increasing the speed or saving energy, yet.
On the other hand, Patent Document 12 has proposed a chemical toner produced by a suspension polymerization method in which granulation is performed in a water phase. This method is a method of obtaining toner particles by adding monomers, a polymerization initiator, a colorant, a mold release agent, etc. to a water phase containing a dispersion stabilizer under stirring to form oil droplets, and then raising the temperature to perform a polymerization reaction. According to this suspension polymerization method, it is possible to simultaneously improve low-temperature fixing property, hot offset resistance, and heat resistant storage property by successfully reducing the particle size of toner particles and homogenizing resin, but a surfactant (a dispersion stabilizer) must be used and there is a problem that electrostatic property is deteriorated due to remaining of the surfactant.
As described above, there have not yet been present toner binders and toners that realize all of low-temperature fixing property, glossiness, and hot offset resistance, and satisfy the flowability, heat resistant storage property, electrostatic stability, grindability, image strength, folding resistance, and document offset property of a toner.